(a) Field of the Invention
The present invention relates to a fluidic angular rate sensor to be mounted on a moving object such as an automobile, an airplane or the like to detect its location by measuring an angular rate, without using an auxiliary signal sent from an external apparatus, e.g., radio waves sent from a stationary satellite.
(b) Description of the Related Art
The structure of a sensor element 1 which constitutes the main part of a known fluidic angular rate sensor is shown in FIG. 6. According to the conventional manufacturing method, two pairs of metal supports 13 are mounted on a ceramic disc 12 which has fluid passage apertures 12a. Fine tungsten or molybdenum wires 14 are mounted in tension over the respective metal supports 13. In this case, very fine wires are used to obtain a high sensitivity and to detect a small angular rate. Such very fine wires cannot be bonded to the metal supports by means of, e.g., spot welding. Instead, ultrasonic bonding has been used wherein as shown in FIG. 7 a gold layer 14a is metal-plated on the wires 14 and on the heads of the metal supports 13 to allow tight contact therebetween, and thereafter gold balls 15 are used to ultransonically bond the tensed wires 14 to the metal supports 13.
The operating principle of the fluidic angular rate sensor having the sensor element 1 as described above is illustrated in FIG. 8. Specifically, the sensor 1 is mounted on a vehicle or the like and is adjusted such that the wires 14 of the sensor are aligned perpendicular to the plane of angular rate sensitivity. A helium gas H is jetted out from a nozzle N. In this condition, if there is no angular rate, the helium gas H goes straight ahead and equally contacts the two wires 14 to maintain the same temperature condition and hence the same resistance value of the two wires 14. If there exists an angular change rate, the helium gas H is biased to one direction (as shown in FIG. 8) so that one of the wires 14 is cooled more than the other to cause a temperature difference and hence a difference in resistance between wires 14. The angular change rate is measured based on this resistance difference.
The sensitive sensor 1 made by the above-described conventional manufacturing method, however, has the following problems.
First, since gold 14a is metal-plated on the wires 14, a relatively large resistance temperature coefficient of, e.g., tungsten becomes lower, resulting in a degraded sensitivity. The resultant temperature coefficient is dependent upon the thickness of the plated gold 14a. Therefore, the temperature coefficient may vary with a manufacture lot so that a correct measurement of angular rate is not possible without adjusting the temperature coefficient of each lot. Second, since the wires 14 are mounted on the metal supports in tension, this tension remains as residual stresses which become a cause of age variations in resistance values of the wires 14. Thus, a long term stable measurement of angular rate becomes impossible, thus requiring frequent maintenance. The residual stresses are present also in the plated gold 14a so that the age variations become more complicated to the degree that the variations cannot be compensated in practice.
In addition to the above problems, the wires, e.g., tungsten wires, made only by a wiredraw process have a fiber-type structure. Therefore, by supplying electric power to a mounted sensor, the wires are heated to undergo re-crystalization which causes variations in resistance.